God's law required that a firstborn son receive the largest share of his father's inheritance (see 21:15–16; 2 Kings 2:9). The custom of giving the firstborn son a double portion is first recorded here but was implied in earlier practice (see Genesis 25:31–34; 27:1–4; 48:8–22).
that son is the firstfruits of his father’s strength: The eldest male child had this special status because he showed that the father could have children, especially a son.